The present invention relates to means for mounting components in electrical and electronic equipment and, more particularly, concerns a standoff for detachably securing chassis and circuit boards to one another.
Initially standoffs were formed of solid metal shafts which were internally bored and threaded at each end so that each end could be mounted to a chassis or circuit board structure by means of a screw going through the structure and into the internal bore of the standoff. The use of metal standoffs produced very secure attachments, but was not entirely satisfactory for a variety of reasons. Not only was the metal part expensive, but considerable labor was involved in threading the screw into the threaded bore of the metal shaft, this resulting in a slow and expensive assembly process. Furthermore, the standoffs could not be used in situations which did not permit access to the reverse surface of the structure so that the screw could be inserted and turned. Finally, the screws tended to become lost during the assembly process.
In the early 60's and 70's there came into use injection molded plastic standoffs--see, for example, U.S. Pat. No. 3,688,635, issued Sept. 5, 1972, and U.S. Pat. No. 3,777,052, issued Dec. 4, 1973--which were inexpensive (as no metal was used), were of one piece fabrication (so that there was no possibility of lost components), and were of a push-on pull-off design (so that the assembly process proceeded rapidly and without regard to whether or not there was access to the rear of the structure). Furthermore, the standoff could be made of an electrically insulative plastic, if desired, so that an electrically insulative connection resulted. The new plastic standoff was not without its disadvantages, however.
The push-on pull-off design of the retaining mechanisms relied essentially on camming actions resisted by friction or resiliency to prevent accidental separation. During shipment or jostling of an assembly constructed with the push-on pull-off plastic standoffs, however, the plastic standoffs tended to snap out just as easily as they had snapped in. Accordingly, it became common practice to include a locking mechanism which used a non-camming action to insure positive holding together of the assembly. Such locking mechanisms require, in addition to the normal movement of the assembly components away from each other, at least one additional action on the part of the person separating the components of the assembly. Typically, such locking mechanisms are of the snap-in type wherein a snap or holding member is deformed during the assembly process and then abruptly allowed to return to its original orientation. One could still simply push the stand-off into the structure, but in order to thereafter separate the standoff and structure, one had to not only pull the two elements apart, but also simultaneously deform the snap or holding member. Unfortunately, while this improvement removed the tendency of the plastic standoffs to snap out inadvertently, such plastic standoffs--whether with or without the lock or snap-in feature--tended to break at critical points of stress and, even when they did not actually break, the plastic tended to creep (that is, lose its resiliency) so that connections made therewith were unstable--i.e., they became loose over time, allowing both lateral and vertical movement of the structure relative to the standoff, if not initially, then over time. Accordingly, the need remains for a standoff which combines the advantages of the metal standoff and the plastic standoff without the diadvantages of either.
It is an object of the present invention to provide a standoff approaching a combination of the strength and security of a metal standoff with the inexpensiveness and ease of assembly of a plastic standoff.